Oxides Stabilized Zirconia

A second class of membranes are described as dense membranes. They may consist of thin plates of metals (Pd and its alloys, Ag and some alloys) or oxides (stabilized zirconia or bismuth oxides, cerates). These membranes are permeable to atomic (for metals) or ionic (for oxides) forms of hydrogen or oxygen and have been studied, especially, in conjunction with chemical... 

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

TBCs consist of two different materials applied to the hot side of the component a bond coat applied to the surface of the part, and an insulating oxide applied over the bond coat. Characteristics of TBCs are that the insulation is porous, and they have two layers. The first layer is a bond coat of NICrAlY, and the second is a top coat of YTTRIA stabilized Zirconia. 

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

J.K. Hong, I.-H. Oh, S.-A. Hong, and W.Y. Lee, Electrochemical Oxidation of Methanol over a Silver Electrode Deposited on Yttria-Stabilized Zirconia Electrolyte, /. Catal. 163, 95-105 (1996). 

T. Chao, K.J. Walsh, and P.S. Fedkiw, Cyclic voltammetric study of the electrochemical formation of platinum oxide in a Pt/yttria-stabilized zirconia cell, Solid State Ionics 47, 277-285 (1991). 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

The stabilized zirconia family of oxides, especially calcia-stabilized zirconia, are solids in which oxide ion conductivity has been increased to the extent that they are widely used solid electrolytes (Section 1.11.6, Section 4.4.5, and Section 6.8). 

A number of oxides with the fluorite structure are used in solid-state electrochemical systems. They have formulas A02 xCaO or A02 xM203, where A is typically Zr, Hf, and Th, and M is usually La, Sm, Y, Yb, or Sc. Calcia-stabilized zirconia, ZrC)2.xCaO, typifies the group. The technological importance of these materials lies in the fact that they are fast ion conductors for oxygen ions at moderate temperatures and are stable to high temperatures. This property is enhanced by the fact that there is negligible cation diffusion or electronic conductivity in these materials, which makes them ideal for use in a diverse variety of batteries and sensors. 

The parent phase is a stoichiometric oxide M02 with the fluorite structure. Substitution of a lower valence cation for Zr4+ is compensated by oxygen vacancies (Section 1.11.6 and Section 4.4.5). Taking calcia-stabilized zirconia as an example, addition of CaO drops the metal to oxygen ratio to below 2.0, and the formula of the oxide becomes Ca Zrj -x02-x. 

Because the potential developed across a stabilized zirconia electrolyte is simply related to the free energy of the reactions taking place in the surrounding cell, the material can be used to measure the free energy of formation of an oxide. (Details of cells and cell types for this task are outside the scope of this book and only principles will be outlined. For information on these techniques see the Further Reading section at the end of this chapter.)... 

Figure 6.15 Schematic cell using a stabilized zirconia electrolyte to measure the Gibbs energy of formation of an oxide MO. The cell voltage, E, is measured under open-circuit conditions when no current flows.

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